docs: def make_otp_response(vres, client_key):
      """
      Create validation response (signed, if a client key is supplied).
      """
      if client_key is not None:
          sig = make_signature(vres, client_key)
          vres['h'] = sig
      # produce "key=value" pairs from vres
      pairs = [x + "=" + ''.join(vres[x]) for x in sorted(vres.keys())]
      return '\n'.join(pairs)